Imaging devices such as optical cameras and scanners are well known in the art and may be used to quickly and easily capture the image of a document for numerous applications. The images may be processed and stored either chemically on photographic paper, or electronically in the form of electrical signals, either analog or digital. As computer use becomes more prevalent, document imaging is becoming more important and widespread. In a typical digital camera or scanner application, the image data signals produced by the camera or scanner may be used by a personal computer to reproduce an image of the object on a suitable display device, such as a CRT or a printer.
A hand-held or portable digital camera is a digital camera which is designed to be hand held and pointed at the object or document being imaged. A hand-held or portable optical scanner is an optical scanner which is designed to be moved by hand across the object or document being scanned. The imaging device, either digital camera or scanner, may be connected directly to a separate computer either by a data cable or wireless data link. If so, the data signals produced by the imaging device may be transferred to the separate computer “on the fly,” i.e., as the image data are collected. Alternatively, the imaging device may include an on-board data storage system for storing the image data. The image data may then be downloaded to a separate computer after the scanning operation is complete by any convenient means, such as via a cable or an optical infrared data link.
Digital cameras are well-known in the art and various components thereof are described in U.S. patent application, Ser. No. 09/189,128, for TWO PIECE SYSTEM FOR DOCUMENT IMAGE CAPTURE of Thomas C. Oliver, U.S. patent application, Ser. No. 09/295,865, for DIGITAL CAMERA WITH INERTIAL POSITION SENSING of David D. Bohn U.S. Pat. No. 4,131,919, U.S. Pat. No. 4,420,773, and U.S. Pat. No. 4,541,010, all of which are hereby incorporated by reference for all that they disclose.
A typical hand-held digital camera or optical scanner may include illumination and optical systems to accomplish imaging of the object. The illumination system illuminates all or a portion of the object, whereas the optical system collects light reflected by the illuminated target region and focuses the reflected light onto the surface of a photosensitive detector positioned within the imaging device. By way of example, the illumination system may include a light source (e.g., a fluorescent or incandescent lamp or an array of light emitting diodes (LEDs)). The optical system may include a lens and/or mirror assembly to direct and focus the image of the illuminated target region along the optical axis of the optical system onto the surface of the detector.
The photosensitive detector used to detect the image light focused thereon by the optical system may be a charge-coupled device (CCD), although other devices may be used. A typical CCD may comprise an array of individual cells or “pixels,” each of which collects or builds-up an electrical charge in response to exposure to light. Since the quantity of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots on an image focused thereon.
The term “image light” as used herein refers to the light that is focused onto the surface of the detector array by the optical system. The image light may be converted into digital signals in essentially three steps. First, each pixel in the CCD detector converts the light it receives into an electric charge. Second, the charges from the pixels are converted into analog voltages by an analog amplifier. Finally, the analog voltages are digitized by an analog-to-digital (A/D) converter. The digital data then may be processed and/or stored as desired.
Portable imaging devices of the type described above are not without their problems. For example, when a portable imaging device is positioned over a document, it is difficult, if not impossible, to perfectly orient the imaging device by hand so that the optical axis is perpendicular to the document. The task is easier with portable scanners which may comprise a flat base to hold against the document. However, as portable scanners become smaller, their bases become increasingly narrow and correct orientation becomes more difficult.
If the imaging device is not held with the optical axis perpendicular to the document face, the resulting image of the document will be stretched and may be blurry and illegible. When the imaging device is not perpendicular, (i.e., is held at an angle other than 90° to the document face), one area of the document is closer to the imaging device's photosensitive detector than the other. Portions of the document which are closer to the imaging device will appear larger, or zoomed in, in the final image. Portions of the document which are farther from the imaging device will appear smaller, or zoomed out, in the final image. The image will also appear compressed along an axis between the closer area and the more distant area. For example, a circle on the document would appear elliptical, or a square on the document would appear rectangular or trapezoidal in the resulting image. Finally, depending upon the depth of field of the imaging device's optical system, portions of the final image may be out of focus.
Digital cameras may be held in place over a document with a fixture such as a tripod or bracket to help hold them in a given orientation. However, it is difficult to properly orient a digital camera even when held in such a fixture if the fixture allows for angular adjustment, as most common tripods do. Some tripods include an air bubble tilt meter, but bubble meters are not very accurate and offer no feedback for large angles when the bubble moves beyond the viewable window. Furthermore, the usefulness and portability of a digital camera requiring a tripod or fixture for document imaging is greatly limited.
A need therefore exists for a system to automatically maintain a vertical orientation of the optical axis in an imaging device, perpendicular to a document positioned below the imaging device. A need further exists for a system allowing the imaging device to be used in non-vertical applications as well as document imaging.